Dinamica di plastiche non galleggianti in zona costiera

The research topic of this work concerns the study of the dynamics of non- buoyant plastic waste particles in coastal zones. The interaction between plastic waste and the marine environment is a complex phenomenon, as it involves a large number of variables. The objective of this study is to understand how the physical characteristics of plastics, hydrodynamic forces and environmental conditions influence their behaviour and dispersion. The work was carried out through an experimental campaign at the Laboratory of Hydraulics and Hydraulic Constructions of the University of Messina, where experiments were conducted on different dynamic behaviours of the particles, such as sedimentation, incipient motion estimation, bed transport analysis, and rising to equilibrium conditions on an sloping plane under the action of regular waves. A supplementary experimental campaign was conducted at the Hydraulic Engineering Laboratory of the Technical University of Denmark, to study the interaction between microplastics and marine obstacles under the combined action of waves and currents. Innovative video-analysis techniques were developed and applied to analyse the results, allowing the movement of the particles to be precisely tracked and the critical parameters governing their dynamics to be quantified. The main results indicate that the dynamics of non- buoyant plastics are influenced by a complex set of factors. These include the physical and geometrical characteristics of the particles (such as size, shape and density), hydrodynamic conditions (intensity and direction of waves and currents) and boundary conditions (type of seabed and presence of obstacles) play a crucial role. In detail, it was found that particles with rounded shapes or low density are more susceptible to wave-induced mobilisation and oscillation than particles with more edged shapes or higher density. Oscillation phases are particularly relevant during the rising of an sloping plane, where additional variables come into play, such as the roughness of the bottom, the proximity to the breaking zone and potential collisions with other elements. Furthermore, the potential for plastics to be trapped by marine obstacles, such as gravel stones, depends not only on the distribution of these obstacles and the geometric and physical properties of the plastics, but also on the combination of forces that mobilise them. In conclusion, the work conducted makes a significant contribution to the existing literature, demonstrating the importance of experimental investigations in understanding the dynamics of plastic litter in coastal areas. These results not only provide a basis for the development of more accurate numerical models, but may also offer practical guidance for the identification of critical areas of accumulation, facilitating targeted actions for the removal of litter and the protection of marine ecosystems.